1. Field of the Invention
The invention relates to a pressure sensor for static and/or dynamic pressure measurement of a liquid, flowable, or gaseous medium, with a housing and with a pressure measuring cell, on the side facing the medium an opening being formed through which one side of the pressure measuring cell is in direct or indirect contact with the medium to be monitored, the pressure measuring cell converting the pressure to be monitored into a proportional measurement signal by means of an electromechanical converter.
2. Description of Related Art
Pressure sensors are used for monitoring and measuring the system pressure in hydraulic and pneumatic applications. Applications of these pressure sensors are for example hydraulic monitoring in machine tools, the plastics industry or the food industry, in which the pressure of various media, i.e. different liquids or gases, is monitored or measured. In this connection, depending on the application there are a host of different versions, the structure and design of the pressure sensors differing depending on the expected maximum nominal pressure of the medium to be monitored.
Known pressure sensors conventionally have a cylindrical pressure measuring cell which consists of a base body and a membrane which are held at a defined distance from one other and which are joined hermetically tight to one another by a connecting material, for example a brazing solder. In addition to pressure sensors in which the base body and the membrane are two inherently separate components, there are also pressure sensors which have a monolithic pressure measuring cell in which therefore the base body and the membrane are made in one piece. In addition, pressure sensors which are also often called fluid sensors generally have an electronic unit in which the measured values delivered by the pressure measuring cell are converted and optionally conditioned and evaluated. Moreover pressure sensors have an electric connection possibility for supply of the pressure sensor with the required energy and as the measurement signal and switching output.
Regardless of the specific configuration of the pressure measuring cell, it holds that the side of the membrane which is in contact with the medium to be monitored, in operation undergoes a deflection which is proportional to the pressure of the medium which can be detected by means of an electromechanical converter which is located on the other side of the membrane.
In the first type of pressure sensors, the measurement principle is the capacitive measurement principle (compare for example DE 44 16 978 C2 or DE 196 28 551 A1, corresponding to U.S. Pat. Nos. 5,824,909 & 5,892,156). The electrode-coated inside surfaces of the membrane and of the base body form a measurement capacitor with a capacitance which is dependent on the sagging of the membrane and thus is a measure of the pressure prevailing on the membrane. In practice, generally pressure measuring cells of ceramic are used, since ceramic pressure measuring cells have high measurement accuracy which remains stable over a very long time. On the side facing away from the membrane, the pressure measuring cell has an electronic circuit which converts the capacitance of the measurement capacitor into a pressure-dependent electrical signal and makes it accessible via electrical connecting lines for further processing or display.
In addition to these pressure sensors which have a capacitance measurement principle, in which the electromechanical converter is therefore two electrodes, there are also pressure sensors which have foil strain gauges (DMS) or pressure sensitive resistors or DMS resistors (for example, DE 102 21 219 A1). For these pressure sensors the DMS resistors are applied to the side of the membrane facing away from the medium, the resistance value of the DMS resistors depending on the sagging of the membrane and thus likewise being a measure of the pressure prevailing on the membrane.
In these pressure sensors or pressure measuring cells thus a separate base body which is located opposite the membrane is not necessary for measurement engineering purposes. There are therefore also pressure sensors or pressure measuring cells with DMS resistors in a monolithic embodiment in which the base body and the membrane are made in one piece. In this connection then the membrane is generally located on the side of the base body facing away from the medium, the base body having a blind hole in the area of the membrane. The base body thus has a pot shape, the bottom of the pot being formed by the membrane and the open side of the pot facing the medium. But in addition it is also possible for the membrane to be located on the side of the base body facing the medium. The pressure measuring cell is either likewise a ceramic measuring cell or a steel measuring cell, especially a high-grade steel measuring cell.
It is common to the known pressure sensors regardless of the specific configuration and the measurement principle which is implemented, that the pressure measuring cell is located in the housing such that it is completely surrounded by the housing or by the housing and a process connection which is connected to the housing. This process connection is used to connect the pressure sensor to a pipe or container which contains the medium to be monitored. If the pressure sensor does not have a separate process connection, the end of the housing facing the pipe or container is itself made as a process connection, for which on the housing then there is either an outside thread or an inside thread for connection to a corresponding coupler or for screwing into a corresponding hole in the pipe or container.
The dimensions, especially the diameter of the pressure sensor, depend on the diameter of the pressure measuring cell used, the outside diameter of the pressure sensor being distinctly larger than the diameter of the pressure measuring cell. If the diameter of the pressure sensor is to be reduced, it is thus first of all necessary to reduce the diameter of the pressure measuring cell which is generally cylindrical. This is achieved in the prior art in that instead of a conventional pressure measuring cell in thick film technology, a pressure measuring cell in thin film technology is used, i.e. a pressure measuring cell in which the electromagnetic converter, especially the DMS resistors are made not in thick film technique, but in thin film technique, and are applied to the measuring cell. In this way the diameter of the pressure measuring cell can be distinctly reduced, but the disadvantage is that the production costs for these pressure measuring cells in thin film technique are distinctly higher than the comparable pressure measuring cells in thick film technique.